1. Field of Invention
Embodiments of the present invention relate to techniques for reducing switching noise affecting a signal. More particularly, some embodiments relate to techniques for reducing noise inserted into a signal by switching circuits in a transmitter driver when a signal is being generated for transmission in a transmission medium. More particularly, some embodiments relate to techniques for operating two parallel switching circuits in a predriver of a transmitter to increase a switching rate for the transmitter and avoid switching at a resonant frequency of the transmitter.
2. Discussion of Related Art
Transmitters that communicate a data signal to a receiver in a transmission medium (e.g., a wire) include various electric circuits to generate the signal. Among the circuits are drivers that each receive an input signal and, using switches, create an output signal that is an amplified version of the input signal. Multiple drivers can be chained together to amplify a signal to a level that it can be transmitted with a desired power to travel a desired distance. A last driver in a chain that outputs the signal from the transmitter (e.g., onto the transmission medium) is termed the output driver, while any drivers preceding the output driver in the chain are termed predrivers.
FIG. 1 shows a transmitter 100 that includes a circuit 102 that is outputting data to a chain of predrivers (also called a predriver horn) 104 that are each switching and amplifying the data signal. The last predriver of the chain 104 in turn provides the amplified data signal to an output driver 106 that generates a signal in a transmission medium 108. Circuit 100 creates two similar signals in the transmission medium 108, a positive and a negative, logically inverted version of the data signal, and thus its data path includes a chain of predrivers for the positive signal and a chain of predrivers for the negative signal and an output driver 106 that has two switches and two outputs.
Each of the drivers includes switches that operate in response to the input data signal. When a signal is switched, oscillations are inserted into the signal that create noise in the supply that degrades the signal. Typically, the oscillations/noise affect a signal for a small period of time before “settling.” Accordingly, the oscillations are typically eliminated before more oscillations are inserted by the next switch of the switches. However, this settling time is impacted by a rate at which the oscillations are being created in the signal, which in the case of a chain of predrivers is the rate at which to the switches are switching. At a particular rate, known as the “resonant frequency,” this settling time will be large, such that some oscillations may exist in the signal at the time more oscillations are created. This can lead to large oscillations and changes in phase that can lead to jitter in the signal communicated in the transmission medium. This jitter can cause data errors at the receiver and interfere with communication.
FIG. 2 illustrates in part (a) the package parasitics of FIG. 1's R-L-C model, showing lumped components on supply and ground, and how this reduced form can be used to determine the resonant frequency ωR and damping factor ζ of the circuit 100 (the damping factor affecting the length of the settling time). FIG. 2 illustrates in part (b) the magnitude and phase of the impedance of the circuit 100 at various frequencies, including the resonant frequency (100 MHz) that creates the most jitter in the signal.
Switches in the predrivers change state each time a bit of the input data signal changes. A 0 bit followed by a 0 bit does not cause a switch, while a 0 bit followed by a 1 bit does cause a switch. FIG. 3 shows such a case, showing that as the data signal Data does not change between cycles of the Clock, the switches do not change state and there is no Switching Current, but if Data does change there is a corresponding Switching Current. As data is being input to the chain of predrivers, a particular sequence of bits may cause the switches of the predrivers to switch at a rate that matches the resonant frequency of the chain of predrivers and causes the oscillations that lead to jitter in the signal.
FIG. 4 shows a graph of this potential jitter for one predriver. For an input signal IN the predriver may generate a range of output signals OP, rather than a single clean signal. Some of these output signals in the range may cause errors at the receiver.
Some techniques have been proposed for avoiding errors that result from jitter. Some of these techniques change the chain of predrivers in a way that changes the resonant frequency to avoid the resonant frequency from being reached. For example, some techniques propose additional decoupling capacitors for the predriver circuit that would counteract the natural inductances of the circuit and lower the resonant frequency to a level that would be below switching frequencies. Other techniques change the circuit in a way that reduces settling time at the resonant frequency in an attempt to avoid the jitter from occurring even at the resonant frequency. For example, some techniques add small resistances that would increase the damping factor and thus decrease the settling to time. These resistances include a DC resistance that shorts to ground to draw some energy away from the oscillating signal path.
One other technique for avoiding jitter includes adding a “shadow” switching circuit to the transmitter. The shadow switching circuit does not contribute to generating a signal in the transmission medium, but rather only acts to affect the chain of predrivers in a way that avoids the resonant frequency of the chain of predrivers from being reached. As discussed above, the resonant frequency may be reached by switches of the predrivers switching and not switching according to a particular bit sequence. The shadow switching circuit in these techniques switches a signal each time a predriver does not switch the signal. In this way, a switch is performed on each clock cycle, rather than dependent on the value of the bits. This maintains the rate of switching as a constant switching on each clock cycle, such that the switching rate does not vary and does not reach the resonant frequency. In these circuits, the clock rate is above the resonant frequency of the chain of predrivers, so switching on each clock cycle causes the switching rate to be above the resonant frequency. In this way, even though the chain of predrivers may be switching according to the resonant frequency, the entire circuit (the chain of predrivers and the shadow circuit) is switching at a frequency well above the resonant frequency.